Multistage axial fan with boundary layer control



Dec. 28, 1954 T. H. TROLLER ETAL MULTISTAGE AXIAL FAN WITH BOUNDARY LAYER CONTROL 5 Shets-Sheet 1 Filed May 12, 1949 92220215: iii? A! J??? QMY NM,\ a E: a, l w\ E .Ub kw Dec. 28, 1954 'r. H. TROLLER ET AL 2,693,129

MULTISTAGE AXIAL FAN WITH BOUNDARY LAYER CONTROL Filed May 12, 1949 5 Sheets-Sheet 2 Dec. 28, 1954 'r. H. TROLLER ET AL 11 i MULTISTAGE AXIAL FAN WITH. BOUNDARY LAYER CONTROL Filed May 12, 1949 5 Sheets-Sheet 3 Dec. 28, 1954 T. H. TROLLER ET AL MULTISTAGE AXIAL FAN WITH BOUNDARY LAYER CONTROL Filed May 12, 1949 5 Sheets-Sheet 4 Fig.5

Dec. 28, 1954 "r. H. TROLLER ET AL MULTISTAGE AXIAL FAN WITH BOUNDARY LAYER CONTROL 5 Sheets-Sheet 5 Filed May 12, 1949 TYPICAL FAN PERFORMANCE OF Axum. FLOW FAN WITH AND WITHOUT BOUNDARY LAYER CONTROL.

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United States Patent MULTISTAGE AXIAL FAN WITH BOUNDARY LAYER CONTROL Theodor H. Troller, Akron, and Chester P. Jenkins, New Philadelphia, Ohio, assignors to Joy Manufacturing Company, Pittsburgh, Pa., a corporation of Pennsylvania Application May 12, 1949, Serial No. 92,852

14 Claims. (Cl. 230-117) This invention relates to axial flow fans, and more particularly to multistage axial flow fans, and especially to means for improving the performance of such fans.

A multistage axial flow fan ordinarily, in the case of a two-stage fan of the character mentioned, comprises two blade-carrying rotors separated by a circumferentially arranged passage which is commonly broken up by a number of appropriately formed straightener vanes, these straightener vanes being provided for the purpose of correcting or removing the whirling motion imparted to the air by the first stage fan and, under some circumstances, as it were, conditioning the air entering the blades of the second stage to diminish or eliminate whirling of the air leaving them. If desired, further straightener vanes can be provided at the side of the second stage fan.

As the air passes through the similar passageways into which the generally annular air flow zone is broken up by the straightener vanes, the air streams tend to have the portions thereof which are closest to the walls which form the boundaries of these multiple streams progressively and distinctly retarded by friction with such walls, and there is accordingly something of a slowing down of the air streams and a reduction in the freedom of air flow into the second stage. It has been found by us that if the relatively thin retarded layers be bled off, as by venting to lower pressure areas, there will result, even though one, two or more per cent of the air entering the first stage is not delivered by the second stage of the fan, a distinct improvement in the performance of the fan, more than justifying the bleeding off of the air, and also more than compensating for its loss. The process of bleeding off this air may be termed boundary layer control and it will be understood that it consists of the removal of slowed-down air near a wall or the walls of the interrupted annulus housing the air stream; and this control may utilize the pressure difference between a point inside the annulus beyond the first stage of the blower, such as, but not necessarily, a point just ahead of the second stage, and some point of lower pressure, to effect the discharge of the retarded layer or layers of the air stream. The air may be bled off inwardly or outwardly from an annular zone, or both, or through the lateral boundary walls which subdivide such annular zone, or in combinations of lateral, inward and outward bleedings and in any event there will be improvements in performance in important particulars, as shown hereinafter in connection with the graphs provided in one of the appended figures. At this point it will suffice to say that among the benefits attained are an extended working range of the blower, increase in efficiency, and an extension of the stable operating range of the fan.

It will thus appear that in its broader aspects the invention consists in the provision of a plural stage, axial flow fan, with the air delivered between the stages through a subdivided annular passage, the subdivisions of which are effected by straightener blades, there being provided adjacent at least one boundary of the annularly arranged flow passages and desirably at a plurality of such boundaries, and preferably, but not necessarily, close to the point of communication of these passages with the succeeding higher stage, vent means to a point of lower pressure, permitting the restricted escape radially-inwardly, outwardly, or bothor laterally, or in varied combinations of the portion or portions of the air streams which are the most retarded.

From the foregoing it will be apparent that an object of our invention is to provide an improved axial flow fan. A more specific object of the invention is to provide an improved axial fiow fan of the plural stage type. A further object of the invention is to provide an improved plural stage fan having means incorporated therein for improving the efficiency thereof by venting the outer, inner or both boundary layers of the air streams as these are about to enter a higher stage. Still another object of the invention is to provide an improved plural stage axial flow fan having blades carried by hub portions at a substantial radial distance from the axis of rotation of the fan rotors and having a subdivided annular zone for the transmission of air from a lower to a higher stage, with provision for the bleeding off of the retarded inner boundary layer of the air stream, desirably, but not necessarily, as such stream is about to enter the higher stage. A still further object of the invention is to provide an improved plural stage axial flow fan having blades carried by hub portions at a substantial radial distance from the axis of rotation of the fan rotors and having a subdivided annular zone for the transmission of air from a lower to a higher stage, with provision for the bleeding off of the retarded outer boundary layer of the air stream, preferably, but not necessarily, as such stream is about to enter the higher stage. Still a further object of the invention is to provide an improved plural stage axial flow fan having blades carried by hub portions at a substantial radial distance from the axis of rotation of the fan rotors and having a subdivided annular zone for the transmission of air from a lower to a higher stage, with provision for the bleeding olf of the retarded inner boundary layer of air and of the retarded outer boundary layer of air, preferably, but not necessarily, as the stream is about to enter the higher stage. Yet a further object of the invention is to provide an improved plural stage axial flow fan having blades carried by hub portions at a substantial radial distance from the axis of rotation of the fan rotors and having a subdivided annular zone for the transmission of air from a lower to a higher stage, with provision for the bleeding off of any wall-retarded boundary layers of air after the same have been materially retarded. Other objects and advantages of the invention will hereinafter more fully appear.

In the accompanying drawings, in which one illustrative embodiment which the invention may assume in practice has been shown, and in which there is presented a series of graphs showing the results of the incorporation of the invention,

Fig. l is a view partially in side elevation and partially broken away to a radial plane, through a plural stage axial flow fan constructed in accordance with the illustrative embodiment of the invention, one of the passages for the flow of air being shown other than as it actually existsnamely as though it were straightfor simplicity of illustration.

Fig. 2 is a vertical transverse section, on an enlarged scale, on the plane of the line 22 of Fig. 1.

Fig. 3 is an enlarged fragmentary sectional view on a radial plane corresponding to the plane of Fig. 1, through the passages and chambers at the upper right hand corner of the intermediate casing shown in Fig. 1.

Fig. 4 is an enlarged fragmentary longitudinal section, with parts in elevation, showing details of the motor cooling passages and the motor which drives the fan blades, this view, similarly to Fig. 1, having certain passages shown other than as they are, for simplicity of illustration.

Fig. 5 is an enlarged developed view on the surface of revolution of the curved section line 55 of Fig. 2.

Fig. 6 is a fragmentary vertical transverse section, on a reduced scale, on the plane of the line 66 of Fig. 4.

Fig. 7 is a multiple graph showing various curves for the purpose of making readily understood the results of the incorporation of the invention.

Referring first to Fig. 1, it will be noted that the twostage axial flow fan 1 which it illustrates includes a base 2 built up of structural shapes and supporting, by means of transverse angles 3 and arcuately cut out supporting plates 4, of which six are shown, a built-up fan and motor housing H including a plurality of portions. At the left hand end of Fig. 1, there will be noted to be a bell mouth section for the guiding of entering air construction.

mentioned and a corresponding opposite flange 16.

'towards 'thefan,--this bell mouth section being designated and bolted, as at 6, to a generally cylindrical section 7 through which entering air passes to the first stage 8 of the fan. A dome-like nose portion 9 having radially extending centering elements 10 is bolted, as at 11, within the section 7 and contains a chamber 12' in which fan blade adjusting means can be located if desired. The

This first stage 8, which is bolted between a flange 'ls'on the housing section 7 and another flange 16 on an intermediate section 17 of the fan, consists of a builtupstructure in the form of channel elements 19 and 20 which cooperate to provide a peripheral wall 21 which conforms in shape to a'zone of a sphere and thus pro- 'vides for the maintenance of close blade tip clearance upon rotary adjustment on their axes of the'fan blades which rotate within the annular passage 22, which is bounded at its outer periphery by the arcuate wall 21 'mentioned. A fan hub 23, mounted in any suitable manner on one end of the shaft of a motor 24 carries radially extending fan blades'25 having shanks 26 rotatably'sup- 'ported in bearings 27 in an annular rim portion 28 of the hub 23. The fan blades draw the air from the chamber within the casing section 7 and discharge it into an annular Zone 30 which lies within the section 17 of the'fan.

The section 17 of the fan is of a somewhat complex The motor 24 has a casing 35, to which field poles 36having associated with them field windings 37 are secured. A rotor including pole pieces 39 having associated windings 40 is rotatable within the motor housing. The motor housing includes at its ends radially extending ring-like members'41 and 42, to which there is suitably secured a circumferential inner wall or housing element 43. To the ends of this wall element there are secured, as at45, closure plate elements 46. The opposite ends of the shafts of the motor extend through these closure plates. The wall element 43 is separated from a relatively outer wall or housing element 44, which is also cylindrical and which carries the flange 16 previously As will appear, the relatively inner and outer wall elements are" the inner and outer bounding walls of air flow passages connecting two fan stages. The outer wall element 44 is maintained coaxial with and in spaced relation to the inner wall el'ement43 by a plurality of straightener vanes 47 which are in the form of elongated, radially low, warped air foils having their leading edges adjacent the discharge side of the first stage of the fan and so extended that their trailing edges extend into rather close adjacency to the second stage of the fan. In the structure specifically illustrated, ten such straightener vanes are shown, but it will be appreciated that the particular number of vanes employed may be widely varied in different fans, and the number is mentioned primarily with a view to explaining why it was impracticable to make true sections corresponding to Figs. 1 and 4 without creation of confusion. Each of the straightener vanes is' formed with a plurality of passages extending through it radially of the fan. These passages will shortly be referred to again and described in some detail. Between the straightener vanes there are, with the construction illustrated, ten passages 50 of a construction correctly shown in Fig. 5. These passages extend from points adjacent the discharge side of the first stage fan to points adjacent the intake side of the second stage fan. There are relatively narrow annular chambers between the wall elements 43'and 44, as illustrated at 52, closely adjacent to the fan sections. '44 there is another curved plate structure 55. This ex- Outside of the outer wall element tends, adjacent each end of the casing section 17, comfpletelyfaround the periphery of the latter, and 'forms a shell with which several of the support elements 4 cooperate The central portion of the plate structure "extends only a little more than halfway around the top sectionof the structure. The plate structure 55, which -may be of built-up construction, is spaced from the outer wall element 44 by four annular plate sections 61, 62, '63 and 64. The plates 61, 62, a portion of the outer wall element 44 andta' portion of the plate structure 55 together bound an annular air flow passage '66. The plates 63 and 64, the outer wall element 44, and the other end portion of the plate 55 bound an annular air flow chamber 67, which, for reasons later explained, communicates through ports 68 with an annular chamber 69 which is formed between the plate section 64 and the flange 16, a circular peripheral element 72 and the end of the outer wall element 44. The flange 16 is suitably tapped at 74 for the cooperation with it of studs or bolts 75, later more specifically described.

Each of the straightener vanes is hollow. Each has its interior divided into three passages 77, 78 and 79. The central passages 78 are, lengthwise of the fan structure, wholly between the annular chambers 66 and 67, and th passages 78 communicate outwardly with a space 80 which lies between the central portion of the plate structure 55 and the outer wall element 44 and which opens freely to the atmosphere at a point somewhat below the horizontal center of the fan structure due to the fact that the central portion of the plate structure 55 extends not much more than halfway around the outer wall element 44, thus providing for the free escape of air at all points below the boundaries 81 of the central portion of the plate structure 55.

Small motor driven, axial flow fans 82 and 83 are mounted on the top of frusto-conical seat members 84 and '85 which have bores which communicate with the annular passages 66 and 67.

The passages 77 and 79 communicate at their radially outer ends respectively with the annular chambers 66 and 67. The communications of the inner ends of the passages 77, 78 and 79 in the straightener vanes will be later described.

The second stage fan is very similar in construction to the first stage. It comprises a rotor 91 having fan blades 92 mounted in sockets 93 formed in a rim portion 94 of the rotor 91. The fan rotor is mounted on the right hand end of a motor shaft 95. The fan chamber is closed on one side by the wall 96. Beyond the fan rotor there is a chamber 97 having an end closure wall 98 and in which blade adjusting mechanism may be arranged if desired. A peripheral wall 99, in the same cylindrical surface with the inner wall element 43, extends from a point adjacent the periphery of the fan rotor to the wall 98, and this is connected by radially extending plate elements 100 with an outer wall portion 101. The plate elements 100 may be formed as and perform the functions of straightener vanes if desired. The bolts 75 previously mentioned extend through a flange 102 connected with the wall portion 101, and through two fan housing members105 and 106 similar in construction to the elements 19 and 20. An air collecting and discharge chamber 108 is secured in position to receive air from the second stage fan and to discharge it to any suitable discharge passage, as at 109.

The motor casing, in addition to the portions mentioned, includes bell-like portions 110 and 111 extending respectively into a chamber 112, which is separated from the first stage fan by a partition element 113, and into a correspondingly located chamber 114 which is separated from the second stage fan chamber by the partition element 46. The circulation of cooling air may now be explained. The fans 82 and 83 draw in air and discharge it into the annular chambers 66 and 67. These chambers communicate through openings 116 and 117 in the outer wall element 44 with the passages 77 and 79 formed near the opposite ends of the straightener vanes. These passages are separated by septa 120 and 121 from the passages 78, which communicate with the space 80 through openings 122 in the outer wall element 44, and which communicate through openings 124 in the inner wall element 43 with an annular chamber 125 which surrounds casing 35. The passages '77 and 79 at their inner ends communicate through openings 126 and 127 with the chambers 112 and 114.

It will be evident that the air which passes to the annular chambers 66 and 67 will pass through the passages 77 and 79 into chambers 112 and 114, and this air will then pass through openings 129 and 130 and other openings of which one is shown at 131, into the motor casing, and after passing over the windings of the motor will pass out through passages 132 in the casing 35 into the chamber 125 and from the latter through the openings 124 into the passages 78, and either directly or through the chamber 80 freely to atmosphere again.

It will be observed that the air within the passages laterally bounded by the straightener blades and inwardly and outwardly by the inner and outer wall elements 43 and 44 is under a higher pressure than the air in the chambers 112 and 114. The annular chamber 69 communicates freely with the annular chamber 67 through the plurality of ports 68. Other ports 142 are provided to connect the annular chamber 69 with the air passages between the Wall elements 43 and 44. These ports, in appropriate number, are spaced circumferentially about the extreme end portion of the outer wall element 44. The size and the spacing of the ports 142 is made such, in the illustrative construction shown, as to bleed off say from /2 to 1 per cent (though these values are not absolutely limiting) of the air flowing between the inner and outer wall elements 43 and 44 just before this air reaches the second stage fan; and because the openings 142 are located at the outer boundary of the streams flowing from the first to the second stage, the air which they will bleed off from the moving air stream will be the most stagnant, or at least the least rapidly flowing, outer boundary layer.

Correspondingly, it may be noted that air may be bled off at the inner boundary of the moving, generally annular air stream. It will be noted that between the rim portion 94 of the second stage fan and the right hand end of the inner wall element 43 there is a narrow annular passage 144. It may also be noted that a plurality of openings are shown formed in a circumferentially extending row at 145 through the closure member 46 (see Figure 2), and these openings communicate with the chamber 114, which communicates, as previously mentioned, with the atmosphere. By properly predetermining the width of the annular passage 144, and more particularly by properly predetermining the number and diameter of the ports 145, the most stagnant inner boundary layer portion of the air stream passing towards the second stage fan may be bled off. Perhaps /2 to 1 percent-though these values are not to be construed as limitingof the air, and specifically the most sluggish inner boundary layer of the moving stream, is bled off at this point. While the bleeding off of the boundary layers or layers at points adjacent the higher stage fan will remove the relatively most retarded air, it is not to be overlooked that beneficial effects may be obtained from bleeding at other points and in other ways. For example, in a fan of the general construction shown, bleed openings might be formed in either or both of the wall elements 43 and 44 at points spaced from the second stage fan for venting the boundary layer or layers inwardly or outwardly or both; and moreover, the radially extending walls of the straightener vanes 47 might also be slotted radially or orificed for the bleeding of retarded air, since these walls also will have a retarding effect on the sides of the air streams traversing the passages 50. Additionally it may be noted that, r

with the general construction shown, narrow slots near the second stage fan, at the junctions of the walls of the straightener vanes 47 with the wall elements, 43 and/or 44 may be employed under some circumstances. Without thereby intending to limit the location of the bleed openings, it may be pointed out, by way of example, that bleeding through the wall element 44 into the passage 67, or into at least some portions, such as the portions adjacent the plate section 63, of the space 80, or bleeding through the wall element 43 into the chamber 114, or into at least the portions of the chamber 125 near the ring-like member 41, or bleeding, as through radially extending slots formed in the walls of the straightener vanes 47 either into the passages 79 or into the portions of the passages 78 which are closer to the passages 79, or bleeding, as through slots or openings at the junctions of the side walls of the straightener vanes with the inner and/or outer wall elements 43 and 44, or any combination of such bleedings, with appropriate flow area restrictions so that the volume or volumes bled off will be duly limited, will produce desirable results, just as, though perhaps in differing degree, will the arrangements specifically illustrated in the accompanying drawings.

From what has been said it will be evident that im; proved results can be obtained by bleeding off any or the retarded boundary layers of air, and accordingly 1t is definitely not to be assumed that our invention is limited to bleeding off of any particular one or ones of such boundary layers. It is also obvious that with fans of different construction it would not be necessary to bleed the air into a motor ventilating stream or conduit, and that any form of communication with a point at which a lower pressure prevails and from which point the air may freely escape, would sufiice. It will also be evident that the invention may be employed with fans with more than two stages, if desired.

It would appear that it would not be necessary to discuss the advantages of the arrangement illustrated and described hereinabove at substantial length, but in order that the advantages of the invention may be observed, Fig. 7 has been included in the drawings for the purpose of showing typical fan performance of an axial flow fan with and without boundary layer control. It will be observed that on this graph abscissas are plotted horizontally in terms of volume of fan discharge in cubic feet per minute, while the ordinates respectively are to be read in terms of discharge pressure, in terms of chiciency, and in terms of brake horsepower input to the fan. Considering first the curves A and B, curve A shows the volume pressure curve of a typical axial flow fan without boundary layer control, while curve B is presented to show the volume pressure curve of the same fan with boundary layer control. The marked improvement in curve B over curve A is obvious. The dip or surge in curve B is much less than in curve A, and the favorable operating range of the fan is extended into the lower volume range. The maximum pressure in the lower volume range is increased. C and D respectively show the mechanical efficiency of a fan without boundary layer control and of the same fan with boundary layer control. Here a marked increase in eificiency, particu larly in the lower volume range, is to be noted. Curves E and F respectively show the relation of brake horsepower to volume, curve E being that for a fan without boundary layer control and curve F that for a fan with boundary layer control. While the brake horsepower is increased for the fan with boundary layer control, the performance is more than correspondingly increased, so that there is a better overall efficiency.

From the foregoing description it will be evident that we have provided, through the bleeding oif of the most retarded layers, so to speak, of the moving air streams between stages, a marked improvement in the performance of the fan.

While there is in this application specifically described one form, with various modifications, which the invention may assume in practice, it will be understood that these are disclosed for purposes of illustration, and that the invention may be modified and embodied in various other forms without departing from its spirit or the scope of the appended claims.

What we claim and desire to secure by Letters Patent is:

1. In an axial flow fan, in combination, a casing section providing inner and outer walls, a fan at one end of said casing section for forcing air longitudinally of the latter between said walls, a fan at the other end of said casing section for receiving air moving between said walls and imparting additional pressure thereto, each of said fans having a rim portion from which blades project and which constitutes in effect a continuation of said inner wall, said casing section having therein between said inner and outer walls a circular series of passages each of a dimension, lengthwise of the casing section, a plurality of times its dimension radially of said casing section and each extending from points adjacent the discharge side of the first fan to points adjacent the intake side of the second fan, and means communicating with the space between said walls in a zone between said fans at a point short of, but adjacent to, said second mentioned fan for venting off to the exterior of said casing section a substantially frictionally retarded boundary layer of the air moving from the first fan to the second fan between said walls.

2. In an axial flow fan, in combination, a casing section providing walls for guiding an air stream, a fan at one end of said casing section for forcing air longitudinally of the latter between said walls, a fan at the other end of said casing section for receiving air moving between said walls and imparting additional pressure thereto, each of said fans having a rim portion from which blades project and which constitutes in effect a continuation of one of said walls, said casing section having therein between said walls a circular series of passages each of a dimension, lengthwise of the casing section, a

plurality of times its dimension circumferentially of said casing section and each extending from points adjacent the discharge side of the first fan to points adjacent the intake side of the second'fan, and means communicating with the space between said walls in a zone between said fans at a point short of said second mentioned fan but nearer to it than to the first fan for venting off to a point exterior to said casing section a boundary layer of the air moving along said walls from the first fan to the sec- --ond fan.

3. In an axial flow fan, in combination, a casing section providing walls for guiding an air stream, a fan at one end of said casing section for forcing air longitudinally of the latter between said walls, a fan at the other end of said casing section for receiving air moving be- 1 tween said walls and imparting additional pressure thereto, each of said fans having a rim portion from which blades pro ect and which constitutes in effect a continuation of one of said walls, said casing section having therein between said walls a circular series of passages eachof a dimension lengthwise of the casing section a plurality of times the dimension transversely of said passage of any bounding wall of the latter, and each passage extending from points adjacent the discharge side of the first fan to points adjacent the intake side of the 'second fan, and means between said fans and in advance of the second mentioned fan for venting off to a point exterior to said casing section concurrently a plurality of boundary layers of the air stream each moving along a gifferent one of said walls from the first fan to the second an.

ing additional pressure thereto, each of said fans having a rim portion from which blades project in directions radial of said fan and which constitutes in effect a continuation of one of said circumferential walls, said casing section having therein a circular series of passages bounded by said air stream guiding walls and each much elongated as compared with the axial dimension of said fans, and each extending from points adjacent the discharge side of the first fan to points adjacent the intake side of the second fan, and means communicating with the air moving between said walls at a point between said fans and in advance of but close to said second mentioned fan for venting off to a space at a pressure below fan discharge pressure thin boundary layers of air moving along different ones of said walls between said fans.

5. In an axial flow fan, in combination, a casing section providing an inner wall and an outer wall, a fan at one end of said casing section for forcing air longitudinally thereof between said walls, a fan at the other end of said casing section receiving air from the latter and imparting additional pressure thereto, said second fan having a rotor having a blade supporting rim closely adjacent to but axially spaced from the end of said inner wall which is nearer said second fan to provide an annular vent opening, and means connecting the radially inner side of said annular vent opening to atmosphere.

6. In an axial flow fan, in combination, a casing section providing inner and outer air guiding walls, a fan at one end of said casing section for forcing air at one pressure longitudinally of said casing section between said walls, a fan at the other end of said casing section for receiving air from between the walls, a motor drivingly connected to said fans and positioned therebetween, a casing enclosing said motor, passage means through said casing for cooling air, means connected with said passage means for delivering air at a pressure lower than said first mentioned pressure to said passage means, and means for venting off a portion of the air at said first mentioned pressure before it reaches the second fan, said means for venting including venting means connecting the space between said walls with said passage means.

7. In an axial flow fan, in combination, a casing section providing inner and outer air guidingwalls, a fan at one end of said casing section for forcing air at one pressure longitudinally of said casing section between said walls, a fan at the other end of said casing section for receiving airfrom between the walls, a: motor drivingly connected to said fans and positioned therebetween, a casing enclosing said motor, passage means through said casing for cooling air, means connected with said passage means for delivering air at a pressure lower than said first mentioned pressure to said passage means, means for venting off a portion of the air at said'first mentioned pressure before it reaches the second fan, said means for venting including venting means opening through one of said walls and means connecting the same with said passage means.

8. In a axial flow fan, in combination, a casing section providing an inner air guiding wall and an outer air guiding wall, end plates extending transversely of said inner wall to provide within the latter a motor enclosure, means communicating with said motor enclosure for venting said motor enclosure to atmosphere, a fan at one end of said casing section for forcing air longitudinally of the latter between said walls, a fan at the other end of said casing section for imposing an additional pressure on the air moved by said first fan, said secondfan having a blade-supporting rim constituting in effect a full diameter extension of one of said Walls, and means cominunicating with the space between said walls and with said motor enclosure for discharging a boundary layer of the air between said walls into said motor enclosure.

9. In an axial flow fan, in'combination, a casing section providing an inner air guiding wall and an outer air guiding wall, end plates extending transversely of said inner wall to provide within the latter a motor enclosure, means communicating with said motor enclosure for venting said motor enclosure, a fan at one end of said casing section for forcing air longitudinally of the latter between said walls, a fan at the other end of saidcasing section for imposing an additional pressure on the air moved by said first fan, and means communicating with the space between said walls and with said motor'enclosure including an opening located between said second fan and said inner wall for discharging a boundary layer of the air between said walls into said motor' enclosure.

10. In an axial flow fan, in combination, a casing section providing an inner air guiding wall and an outer air guiding wall, end plates extending transversely of said inner wall to provide within the latter a motor enclosure, venting means for said motor enclosure communicating with the latter, a fan at one end of said casing section for forcing air longitudinally of the latter between said walls, a fan at the other end of said casing sectionfor imposing an additional pressure on the air moved by said first fan, and means communicating with the space between said walls and with said motor enclosure and including an opening between said second fan and the end plate adjacent said second fan for discharging a boundary layer of the air between said'walls into said motor enclosure.

11. In an axial flow fan, in combination, a casing section providing an inner air guiding wall and an outer air guiding wall, end plates extending transversely of said inner wall to provide-within the latter a motor enclosure, venting means for said motor enclosure comunicating with the latter, a fan at one end of said casing section for forcing air longitudinally of the latter between said walls, a fan at the other end of said casing section for imposing an additional pressure on the air moved'by said first fan, and means, including an opening located between said second fan and said inner wall and openings through the adjacent end plate, for discharging aboundary layer of the air between said walls into said motor enclosure.

12. In an axial flow fan, in combination, a casing section providing an inner air guiding wall and an outer air guiding wall, end plates extending transversely of said inner wall to provide within the latter a motor enclosure, venting means for said motor enclosure communicating with the latter, a fan at one end of said casing section for forcing air longitudinally of the latter between said walls, a fan at the other end of said casing section for imposing an additional pressure on the air moved by said first fan, and means, including an opening located between said second fan and the end plate adjacent said second fan and openings through said adjacent end plate, for discharging a boundary layer of the air between said walls into said motor enclosure.

13. In an axial flow fan, in combination, a casing comprising inner and outer wall elements and intermediate straightener vanes extending between said wall elements, said wall elements and straightener vanes providing bounding Walls for a plurality of air flow passages, a fan at one end of said casing section for forcing air longitudinally through said passages, a fan at the other end of said casing section for receiving air from said passages and imparting additional pressure thereto, and means opening through the outer one of the bounding walls of said passages for bleeding off a relatively stagnant boundary layer of air.

14. In an axial flow fan, in combination, a casing comprising inner and outer Wall elements and intermediate straightener vanes extending between said wall elements, said Wall elements and straightener vanes providing bounding Walls for a plurality of air flow passages, a fan at one end of said casing section for forcing air longitudinally through said passages, a fan at the other end of said casing section for receiving air from said passages and imparting additional pressure thereto, and means, including at least one passage opening through one of the bounding walls of said passages and communicating with the boundary layer of air moving along said wall, and at least one generally oppositely opening passage communicating with the opposite boundary layer of air, each of said passages last recited communicating with the space between said wall elements and intermediate said fans, for bleedlng otf relatively stagnant boundary layers of air moving along said walls.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Re. 20,668 Seelig Mar. 8, 1938 1,932,231 Schmidt Oct. 24, 1933 2,046,737 Gosslau July 7, 1936 2,294,586 Troller Sept. 1, 1942 2,298,576 McElroy, et al. Oct. 13, 1942 2,339,602 Hagen Jan. 18, 1944 2,344,835 Stalker Mar. 21, 1946 2,405,768 Stalker Aug. 13, 1946 2,407,807 Bentley Sept. 17, 1946 2,411,124 Baumann Nov. 12, 1946 2,520,697 Smith Aug. 29, 1950 FOREIGN PATENTS Number Country Date 504,214 Great Britain Apr. 21, 1939 586,710 Great Britain Mar. 28, 1947 619,722 Great Britain Mar. 14, 1949 

